In vertebrates, all trunk muscles, which includes the body and limb muscles, are
derived from muscle progenitor cells originating in somites. Somites are
segmentally repeated embryonic epithelial balls of cells that bud off from the
pre-segmented paraxial mesoderm in a rostral -caudal progression along either
side of the neural tube (Brand -Saberi et al., 1996, Dequeant and Pourquie,
2008, Pourquie, 2011) . During embryonic development, somites mature and
differentiate into a variety of distinct tissues: a) the sclerotome, that gives rise
to the vertebrae bone, connective tissue and cartilage surrounding neural
structures; b) the dermomyotome, that gives rise to muscle progenitors, dermis
and angiogenic cells; c) the syndetome, from which axial tendons are derived
and d) the myotome, which is where muscle differentiation first occurs in the
embryo.
Limb muscles originate from somites only at limb -level and the formation of
muscle involves several steps: migration, proliferation and differentiation
(Duprez, 2002). At limb level, progenitor cells at the ventro-lateral lip of the
dermomyotome delaminate and migrate into the limb bud. Delamination and
migration occurs when a signal from the limb bud, HGF (hepatocyte growth
factor)/SF (scatter factorL produced by mesodermal cells from the limb bud,
interacts with C-met, a tyrosine kinase receptor expressed in the muscle
progenitor cells localised at the ventro- Iateral lip (Dietrich et al., 1998). The
progenitors migrate to two distinct regions in the limb: the dorsal muscle mass
and the ventral muscle mass. Only at these two regions do the progenitor cells
activate the myogenic programme and express muscle-specific genes such as
the myogenic regulatory factors (MRFs).
At transcriptional level, there are four key myogenic genes that have been
implicated to be important for trunk muscle formation. These are the MRFs, a
group of four basic helix-loop- helix transcription factors which includes MyfS,
MyoD, myogenin and MRF4 (also known as Myf6). Although the first MRF was
described back in 1987 (Davis et al., 1987L and the first in situ hybridisation
experiments were performed using radioactive -labelled probes (Pownall and
Emerson, 1992), the expression profile for the MRFs during embryogenesis was
not comprehensive and remained fragmented in both chick and mouse.